A [2010] paper in the Journal of Physiology by Robert Fitts of Marquette University suggests the reduction in the capacity for work after six months in space can exceed 40 percent, which would temporarily reduce the performance of a returning astronaut to that of an 80-year-old. The study suggests more effective exercise techniques are required to keep astronauts in shape during long missions.

Fitts said bed-rest studies show resistive exercise can offset the decline. Astronauts aboard the International Space Station typically spend 45 minutes to an hour or so every day actually exercising, either pedaling a stationary bike, jogging on a treadmill, held down by a harness, or using resistive devices. Counting preparation time, crew members devote two hours a day to getting the required exercise done.

The ARED, one of the latest exercise machines aboard the station, works like a high-tech gym to permit effective weightlifting with up to 600 pounds of load, using flywheels to simulate the inertia weight lifters must overcome.

Science fiction writers have long urged space agencies to think about astronaut fitness on long space voyages. In his 1953 novel Space Tug, Murray Leinster writes about a gravity-simulator harness:

"When we got back," Joe told Brown, "we were practically invalids. No exercise up here. This time we've brought some harness to wear. We've some for you, too..."
Joe got out the gravity-simulator harnesses. He showed Brent how they worked. Brown hadn't official instructions to order their use, but Joe put one on himself, set for full Earth-gravity simulation.

He couldn't imitate actual gravity, of course. Only the effect of gravity on one's muscles. There were springs and elastic webbing pulling one's shoulders and feet together, so that it was as much effort to stand extended—with one's legs straight out—as to stand upright on Earth. Joe felt better with a pull on his body.
(Read more about Leinster's gravity-simulator harness)

Heinlein fans recall that in his 1966 novel The Moon is a Harsh Mistress, the loonies heading toward Earth need to start exercising to survive the harsh gravitational field of our planet.

Had to squeeze in hours of heavy exercise, wearing weights, and dasn't arrange permission to use centrifuge at Complex, one used by earthworm scientists to stretch time in Luna...

Exercising without centrifuge is less efficient and was especially boring because did not know there would be a need for it...

Many sf fans also recall the scene in the 1968 film 2001: A Space Odyssey in which Frank Poole gets some exercise by jogging within the turning living space of the Discovery space craft.